Normally you would say “a citizen of the world” in singular.
No, indefinite as in not a particular thing.
Normally it’s simple.
Like if you say “I’m going to the pond behind the house”, then I know there is just one pond behind your house and you’re going to that one. Because you used the definite article, the.
If you say “I’m going to a pond”, then I don’t know which pond you’re going too (and maybe you don’t either). Because you used an indefinite article (see what I did there?).
Sometimes it’s more complicated.
If you say “I’m reading a book” it doesn’t really matter which book, just that you’re reading one. But of course you know which book you’re reading. You’re just not emphasizing that part. A book = any book, not one particular book.
If you say “I’m reading the book” it’s assumed the person you’re talking to knows which book it is, and it is that specific book. For instance you could be reading Alice in Wonderland, your friend knows that and you would be referring to that specific book.
Sometimes you use the definite article even though you don’t have one particular thing in mind. For instance, normally you’d say “I’m going to the cinema” whether you know which cinema you’re going to or not. And you would say “I play the violin” even though you can play any violin. This depends on the noun, in this case cinema and violin. I’m not sure what the rules are or if there are any… Except that instruments are normally referred to with the definite article. “Do you play the piano?” is correct. “I always play a guitar at 2 am” sounds strange.
And sometimes you skip the article entirely, as in “I’m going to school” and “I’m going to work”.
Sorry, this is somewhat incorrect. I would know there’s just one pond behind the house because you used the definite article and the noun is in singular form. If it had been in plural form there would be multiple ponds. And with the first sentence (singular) there could be other ponds further away but there is just one which is in the vicinity of the house and behind it
My English isn’t perfect but I think what I wrote here is correct. Now.